Hitherto, as a semiconductive materials, so-called various compound semiconductors mainly composed of Si or Ge have been known. On the other hand, it is known that a certain kind of metal oxide shows a semiconductivity. However, since these materials are generally prepared by a dry film-forming process, it is difficult to obtain a device having a large area and also the cost for thereof is high.
On the other hand, recently, semiconductive materials comprising organic material have actively developed and have been practically used in some field. In particular, an organic electrophotographic photoreceptor giving an excellent result has been obtained using a macromolecular polymer doped with a low molecular weight compound such as a transporting agent.
A semiconductive material composed of an organic material has many advantages that a semiconductor having a large area can be easily produced, the cost thereof is low, etc. However, such an organic semiconductive material has disadvantages that the semiconductivity is liable to be influenced by moisture absorbed therein and impurity ions existing in a macromolecular polymer, and the semiconductive material is inferior in weather resistance, hardness, and abrasion resistance.
Also, recently, a process of producing metal oxide semiconductors by a sol-gel process is proposed and it becomes possible to obtain a device having a large area at a low cost. However, the metal oxide showing semiconductivity produced by such a process difficultly forms a uniform film and, on the contrary, a metal oxide capable of easily forming a uniform film produced by the process shows an electrically insulating property.
On the other hand, it is proposed in Journal of Applied Physics, 58, 9, 1, p. 3559 (1985) to obtain a semiconductive material by doping a metal oxide capable of forming a uniform film with an organic molecule, but in this case, as the organic molecule being used for doping, an alcohol-soluble or water soluble organic compound only can be used and hence the application thereof is limited to a narrow field of art.
A sol-gel process is a process of hydrolyzing a metal alkoxide at normal temperature to form a sol and after proceeding the reaction for gelling the sol, calcining the gel at low temperature, and various proposals have been made for forming functional materials having various functions using the conventional sol-gel process.
For example, there are a process of producing a composite oxide mixing a metal alkoxide having two or more kinds of metal oxides as described in JP-A-58-25434, JP-A-61-163128, and JP-A-63-123838 (the "term" as used herein means an "unexamined published Japanese patent application"), and Kagaku Kougyou (Chemical Industry), 12, 1007 (1986), a process of adding a metal ion or a metal salt to an alcohol solution or an aqueous solution of a metal alkoxide as described in JP-A-60-51622, JP-A-62-143880, and JP-A-62-143881, a process of adding a fine powder or microcapsules to an alcohol solution or an aqueous solution of a metal oxide as described in JP-A-62-100428 and JP-A-63-2618, and a process of forming a dry gel and then impregnating the gel with a solution containing a metal ion, etc., as described in JP-A-60-51622 and JP-A-61-163124.
Furthermore, on materials being used for doping the use of organic compounds has been also investigated in place of using metal compounds as described in J. Appl. Phys, 58, 9, 1, p. 3559 (1985), J. Non. Cryst. Solid, 82, p. 103 (1986), and J. Phys. Chem., 88, p. 5956 (1984).
However, in these processes described above, alcohol-soluble or water-soluble materials only can be used as the doping materials and other more useful functional materials than the aforesaid materials can not be used as the doping materials.
Also, electrophotographic photoreceptors using organic photoconductors have been variously investigated owing to the advantages thereof that they cause no pollution, they can be produced with high producibility and they can be produced at low cost. However, organic photoconductors have such disadvantages that the organic photoconductor generating electric charges by adsorbing visible light is poor in charge retentivity, while the organic photoconductor having good charge retentivity and excellent film-forming property generally scarecely shows photoconductivity by visible light.
For solving the problem, it has been practiced to employ a double layer type photoconductive (photosensitive) layer having a layer structure of separating each function into a charge generating layer of generating electric charges by absorbing visible light and a charge transporting layer of transporting the charges, and also with respect to the charge generating agents (or materials) and charge transporting agents (or materials), various materials have been proposed. For example, as charge transporting agents, there are known various materials such as amine compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stilbene compounds, carbazole compounds, etc.
Hitherto, as a binder resin for the charge transporting layer, high molecular resins such as polymethyl methacrylate, polyester, polycarbonate, etc., as disclosed in JP-A-57-4051 are generally used.
These high molecular resins show good compatibility with various charge transporting agents and give good electrostatic characteristics but are weak to external mechanical forces owing to the low hardness, which results in restraining the life of the electrophotographic photoreceptor using such a binder resin because of the formation of su face abrasion and scratches.
On the other hand, recently, a process of forming a functional film by doping an organic compound in the case of synthesizing a metal oxide ceramic using a metal alkoxide by a sol-gel process has been proposed and the film formed by the aforesaid process has a high hardness and has an excellent abrasion resistance.
However, in the case of applying the film formed using the metal alkoxide by a sol-gel process, there is a problem that the obtained film has a high hardness and an excellent abrasion resistance but a charge transporting material is not compatible with the layer and a charge transferring function can not be imparted to the film, if the film is applied to the electrophotographic photoreceptor. Thus, the film can not be utilizing as a materials for forming a charge transporting layer of an electrophotographic photoreceptor.